The present invention relates to conductive polymer compositions containing a polymer and a carbon black with specified properties as well as to the use of these conductive polymer compositions in articles for such uses as automotive applications and the like. The present invention further relates to a method of electrostatic painting of an article using these conductive polymer compositions and to the resulting painted article.
Polymeric materials offer several advantages over metals as a material for automotive applications, and consequently are becoming a material of choice for many automotive components. For example, polymeric materials are preferably used for almost all of the components of an automotive fuel system, such as the fuel inlet, filler neck, fuel tanks, fuel lines, fuel filter, and pump housings. Many of these polymeric compounds, however, are non-conducting materials.
Today, automobiles contain more and more electronically operated devices, such as anti-lock brake systems (ABS), electronic fuel injection, satellite based global positioning systems (GPS), and onboard central computers. In order to ensure the safe operation of all of these devices, polymeric materials which provide electrostatic discharge protection and electrostatic dissipative (ESD) properties to automobile parts such as the internal trim, dashboards, panel, seat fibers, switches, and housings are needed.
In addition, electrostatic painting (ESP) is often used to prepare the coated articles for automotive applications. In ESP, a paint or coat is ionized or charged and sprayed on the grounded or conductive article. The electrostatic attraction between the paint or coating and the grounded article results in a more efficient painting process with less wasted paint material and more consistent paint coverage for simple and complex shaped articles. However, polymeric materials that are used in the automotive industry for superior corrosive properties and reduced weight property are typically insulative and non-conducting.
In electromotive coating processes, an electrical potential is used between the substrate being coated and the coating material in order to provide an efficient painting process. In more detail, a paint or coating is charged or ionized and sprayed on a grounded article. The electrostatic attraction between the paint or coating and the grounded, conductive article results in a more efficient painting process with less wasted paint material. Furthermore, an additional benefit of the process is a thicker and more consistent paint coverage. When articles fabricated from metals are painted, the metal which is inherently conductive, is easily grounded and efficiently painted. However, with the use of polymeric materials in the manufacture of many articles, especially automotive applications, the polymers are insufficiently conductive or not conductive at all and therefore do not obtain satisfactory paint thickness and coverage when the article is electrostatically painted. In an effort to overcome this difficulty, compositions containing conductive fibers have been used as well as the use of ion-conductive metal salts. In addition, U.S. Pat. No. 5,844,037, which is incorporated in its entirety by reference herein, provides a mixture of polymers with an electrically-conductive carbon. As shown in that patent, preferably low amounts of electrically-conductive carbon such as from 0.1 to 12% by weight, is used in combination with an amorphous or semi-crystalline thermoplastic polymer and a second semi-crystalline thermoplastic polymer having a different degree of crystallinity. As indicated in the '037 patent, preferably the carbon black has a nitrogen surface area of at least 125 m2/g, and more preferably at least 200 m2/g. Furthermore, the carbon black preferably has a DBPA of at least 75 cc/100 g and more preferably at least 100 cc/100 g. In the industry, typically high surface area carbon blacks have been used in the electromotive coating process since there was an understanding in the industry that high surface area carbon blacks would permit lower amounts of carbon black to achieve the desired conductive article. However, the high surface area carbon blacks are quite expensive and the use of lower amounts of carbon black, while possibly having benefits, also leads to certain detriments due to the low filler content in the article.
Accordingly, the present invention solves this problem by providing low surface area carbon blacks which are capable of being used in high loading amounts and which permit an excellent balance of properties which were not provided by previous attempts in this area.
Certain types of carbon black have been incorporated into polymer compositions to render resistive polymers electrically conductive. The degree of electrical conductivity imparted by a specific carbon black is related to its physical and chemical properties. This has been particularly useful in the preparation of polymer compounds for the cable industry.
Carbon black can also have an effect on the physical and mechanical properties of polymers, which, in turn, can have an effect on the operational lifetime of articles prepared from them. For example, any loss of mechanical properties could lead to premature failure of the parts under normal conditions of operation. Thus, the amount and type of carbon black used to impart improved conductivity to a polymer composition must be balanced with its effect on the overall properties of the polymer composition. Further, some carbon blacks, due to their size and chemistry, are difficult to disperse into polymeric compounds. Poor dispersibility can also lead to poor polymer properties and appearance. Additional mixing of the compound may improve the dispersion quality, but this additional working can also have a detrimental effect on the properties.
U.S. Pat. Nos. 5,902,517, 6,156,837, 6,086,792, 5,877,250, 5,844,037, and 5,484,838, as well as U.S. patent application Ser. No. 09/728706, each incorporated in their entirety by reference, relate to carbon blacks and semiconductive or conductive polymer compositions and articles. However, there remains a need to provide conductive polymer compositions using carbon blacks having high compound conductivity while at the same time having levels of toughness, stiffness, smoothness, tensile properties, etc. that are acceptable for use in automotive applications.